JPEG: allow Chroma quality to be set independently

Hello,
I'm really missing this feature from Ulead Smartsaver 3, in this application Chroma quality could be reduced independently (this is different from Chroma subsampling), there was a switch to unlink Overall quality and Chroma quality, this way you could quantize CbCr components more aggressively while preserving Y'(Luma).

http://cryopng.free.fr/uss/chroma-sample0.png
(Chroma quality is dropped to 0 Overall quality is still 85, resulting compressed file size 29,313)
In this extreme case the picture is still sharp only colors exhibit the usual JPEG compression artifact.

http://cryopng.free.fr/uss/chroma-sample85.png
(Chroma quality back to 85 Overall quality is dropped to 0, resulting compressed file size 9,076)
This time the picture looks like a mosaic but on the other side the colors are those expected.